A brushless motor exhibiting high efficiency can be used as a drive source for a power tool. However, the brushless motor requires a motor driver circuit which is not required in case of an ordinary motor having a brush. In case of a power tool having a brushless motor, the motor driver circuit is accommodated in a power source box additionally provided between a housing of a power tool and an external power source as shown in FIG. 1 of publication No. 2008-504136 which is a published Japanese translation of a PTC text. If a plurality of power tools each having a brushless motor are connected to an identical external power source, each power source box is moved in accordance with the movement of the user, so that power source boxes are bumped against one another other causing damage to the power source boxes.
To avoid this problem, Japanese Patent Application publication Nos. 2007-283447 and 2006-297532 propose accommodation of a circuit board of a motor driver circuit within a housing of a power tool. More specifically, Japanese Patent Application publication No. 2007-283447 discloses a cordless impact driver where an electrical circuit board which is a part of the motor driver circuit is provided between a grip portion and a battery. Further, Japanese Patent Application publication No. 2006-297532 discloses a cordless impact driver where a control board which is a part of the motor driver circuit is accommodated in a handle portion. Furthermore, Japanese Patent Application publication No. 2008-173716 discloses a cordless hammer drill where a FET board which is a part of the motor driver circuit is accommodated within a space positioned above a brushless motor accommodation portion.
However, the layout of the motor driver circuit as described in these JP publications provides drawback in case of a power tool employed by hand-gripping a motor accommodating portion of the power tool such as a disc grinder. First, the circuit board has a portion located radially outward of the brushless motor. Therefore, a diameter of the motor accommodation portion must be increased to accommodate the circuit board therein. Consequently, the user cannot easily grip the motor accommodation portion to lower operability.
Second, the motor driver circuit includes a plurality of switching elements such as field-effect transistors (FET). These FETs release greater amount of heat in comparison with other electronic components because of flowing of driving current and high speed switching operation. Therefore, cooling to the FETs must be taken into consideration if the motor driver circuit is to be accommodated within the housing of the power tool, and further, efficient layout of the motor driver circuit board must be required. Particularly, in case of a high output power tool such as a disc grinder, AC power source is employed instead of battery power source, and accordingly, large size FETs are used. If six FETs are arrayed in a circumferential direction as described in JP 2008-173716, a diameter of a circumference must be increased, causing the housing to be bulky.